A group III nitride semiconductor typified by gallium nitride (GaN) has a wider bandgap, a faster saturation velocity, and a higher breakdown electric field than those of silicon (Si) and gallium arsenide (GaAs). Therefore, transistors which include the group III nitride semiconductor as material and are capable of providing high frequency and high power, such as a Field Effect Transistor (FET), have been actively developed.
A crystal having a wurtzite structure is used for the group III nitride semiconductor FET for its ease in crystal growth. For example, when a heterojunction interface is produced using AlGaN and GaN as the group III nitride semiconductor, a charge is generated on the interface by piezoelectric polarization and spontaneous polarization. Thus, two-dimensional electron gas is generated near the heterojunction interface, without doping an impurity. In the group III nitride semiconductor FET, the two-dimensional electron gas is used as a channel that is a path for electrons.
The following describes a structure of a general nitride semiconductor FET with reference to FIG. 10. First, a GaN channel layer 1102 and, an AlGaN barrier layer 1103 are sequentially grown on a substrate 1101, with the metalorganic chemical vapor deposition (MOCVD). At this time, as described above, in the GaN channel layer 1102, a channel 1104 formed by two-dimensional electron gas is generated near the heterojunction interface of the AlGaN barrier layer 1103 and the GaN channel layer 1102 as indicated by a dotted line in FIG. 10.
Next, a gate electrode 1108 and an ohmic electrode 1107 that is used as a source electrode and a drain electrode are formed on the AlGaN barrier layer 1103. At this time, the AlGaN barrier layer 1103 serves as a great potential barrier against the electrons, thereby preventing the electrons to flow from the ohmic electrode 1107 to the channel 1104. Accordingly, contact resistance in the ohmic electrode 1107 increases.
Therefore, as shown in FIG. 11, a channel 1204 is exposed by forming an ohmic recess portion 1205 by dry etching from a surface of the AlGaN barrier layer 1203, penetrating the AlGaN barrier layer 1203, to the GaN channel layer 1202, so that an ohmic electrode 1207 is in direct contact with the channel 1204. It is already disclosed that the ohmic electrode 1207 formed in such a manner is effective in reducing the contact resistance (see Patent Literature (PTL) 1, for example).
Furthermore, the ohmic recess portion 1205 is formed from the surface of the AlGaN barrier layer 1203, penetrating the AlGaN barrier layer 1203, to a position deeper than the channel 1204 in the GaN channel layer 1202, by dry etching. It is already disclosed that increasing the area of the lateral side of the ohmic recess portion 1205 is effective in reducing the contact resistance (see PTL 2, for example).